BERLIN — Germany's opposition has ousted Chancellor Angela Merkel's party from the mayor's office in the capital of a traditional battleground state – a blow ahead of national elections in September.

Center-left Social Democrat Sven Gerich won election in the western city of Wiesbaden in a runoff ballot Sunday with 50.8 percent of the vote, defeating incumbent Helmut Mueller of Merkel's conservative Christian Democrats.

Wiesbaden is the capital of Hesse state. Merkel's party has run the state, which has swung between left and right over the decades, since 1999, and hopes to extend that run in a state election Sept. 22 – the same day Germany elects a new Parliament.

The result extends a run of poor results in major cities for Merkel's party, which lost Frankfurt and Stuttgart last year.